Modern automotive air conditioning system condensers are generally all aluminum designs, in which aluminum flow tubes, fins and refrigerant inlets and outlets are all brazed together concurrently. Braze cladding material on the outer surface of the various parts melts in a braze oven and is drawn into the various surface to surface interfaces between components to create solid, leak proof joints. In general, braze technology has advanced to the point where, if the edge to edge or edge to surface interfaces between various parts of the manifold can be evenly held, then melted braze material will be drawn evenly and adequately into those interfaces to form solid, leak proof joints. The manufacturing challenge, then, is to hold the interfaces "to print."
One common brazed condenser design is the so called serpentine, in which only one (or two) very long flow tubes wend sinuously back and forth over the entire surface area of the condenser. The one or two tubes have only two open ends each, each of which opens to a small inlet and outlet fixture. The serpentine design presents very few potential external leak points, but is limited as to how closely the various runs of the tube may be spaced, since the tube can not be bent too tightly. The other basic condenser design uses two long, spaced apart manifolds, and a plurality of short, straight tubes, each of which opens through a slot into each of the manifolds. Refrigerant is fed in and out of the tube ends through the common manifolds. While a thinner tube may be used (since it doesn't have to be bent), there are clearly many more potential leak points, two for each tube. Furthermore, most manifolds are two piece designs, formed from two split sections secured together at abutted longitudinal edges. One manifold section is slotted, to admit the ends of the flow tubes, and generally referred to as a header plate, which the other section may be referred to as a manifold tank. The abutted edges represent external seams, which must be sealed against the internal pressure. In addition, each end of the manifold must be plugged with a suitable brazed end cap or plug.
Another issue with two piece manifolds is the use of internal separators/ and or end caps. These are basically the same type of internal structures, being a close fitting plug that is brazed closely within the interior of the manifold, with an outer edge that must make a leak free joint with the inner surfaces that it contacts. Each manifold must have two such structures, one at each end, which are referred to as end caps. In addition, if it is desired to "multi-pass" the condenser, there must be at least one such structure somewhere between the two end caps in at least one manifold. So used, such structures are generally referred to as separators or baffles. If only one is used, in just one tank, with an inlet above and an outlet below, then a two pass flow pattern is created. The addition of a separator in the other tank creates a three pass pattern, and so one. Separators located intermediate the end caps obviously do not present problems of external leaks if they are inadequately brazed, although reduced efficiency results if refrigerant leaks internally past an inadequately brazed separator.
While a poorly brazed internal separator creates no external leaks in and of itself, an only recently recognized problem is the potential effect that a common method of separator installation can have on the longitudinal seam integrity. The problem results from the means used to temporarily, mechanically hold the separator in place between the header and manifold tank before the braze operation is completed. Current designs generally use a pair of aligned recessed "pockets" or retention grooves in the manifold tank and the header plate, within which the edge of the separator sits and is held. When the header plate is clinched to the tank, the separator is securely sandwiched and held in place between the aligned grooves. An example may be seen in U.S. Pat. No. 5,329,995, where the inner surfaces of the header and manifold tank (and the retention grooves) have differing diameters, requiring that the separator have a "notched" shape to match. U.S. Pat. No. 5,607,012 improves upon that design by forming the two grooves on a common diameter, thereby allowing the separator to be a simple circular disk, with no preferred orientation.
A recent improvement of the round separator design referred to above has eliminated one of the pockets or retention grooves entirely, leaving only the groove in the manifold tank. The inner surface of the header plate, rather than being grooved, is final formed with an accurate cylindrical surface, so as to closely match the outer edge of the separator, with a close tolerance interface. At installation, the manifold tank is oriented to open upwardly, and the separator/end caps are set into the retention grooves, which hold them in place. Next, the header plate edges are abutted to the manifold tank edges, and the two are clinched together. If the header plate and the manifold tank are both formed accurately and to print, then the cylindrical inner surface of the header plate and the bottom surface of the manifold tank will lie on a common circle. That common circle, in turn, will make a close controlled interface, all the way round, with the circular (annular) perimeter edge of the separator, so that a good braze joint will form. In addition, the longitudinal edges of the header plate and manifold tank will make close, accurate contact all along their length, to form an accurate interface and braze seam.
The only recently recognized problem referred to above is the effect that the method of forming the retention groove in the manifold tank (or in the header plate, in cases where there is one) has on the longitudinal edge. As can be seen in FIG. 1, the concave (semi cylindrical) inner surface of the manifold tank 10 is initially smooth, and extends for approximately 180 degrees up to a pair of spaced longitudinal edges 12, which are approximately 0.12 inch wide. Standing up from the edges 12 is a pair of crimp flanges 14. Separator 16 is a simple circular disk, with an outer edge 18 of pre determined diameter D, which is about 0.74 inch the embodiment disclosed, as well as approximately 0.08 inch thick. The circular outer edge 18 represents a nominal perimeter that the separator 16 would ideally occupy within tank 10, as indicated by the dotted line in FIG. 2. The radius of the majority of the inner surface of tank 10 is approximately 20 thousandths of an inch less than separator edge 18. As it approaches the longitudinal edges 12, however, the inner surface departs radially inwardly from it's majority radius to create a narrowed trough of width W, which is about 0.64 inch, as measured between the edges 12. In effect, the tank edges 12 are widened as the space between them is narrowed.
Referring next to FIGS. 2 through 4, a coining punch 20 has a semi circular working edge 22 with a diameter and thickness substantially equal to the separator edge of separator 16. The radius of edge 22 is actually very slightly larger than separator edge 18, approximately two thousandths of an inch greater, in order to create an ideal radial brazing clearance of the same size. Tank 10 is supported in the upward opening orientation shown and punch 20 is moved forcefully in a direction normal to, and centrally between, the edges 12, at each location where a separator/end cap 16 is to be installed. This would be at least near each end of each tank 10, to plug the ends, and anywhere else where a flow division point was needed. Ultimately, the punch 20 is stopped when its working edge 22 reaches the nominal perimeter represented by the dotted line in FIG. 2. Since its diameter is greater than the inner surface of tank 10, the punch edge 22 is forced into the inner surface of tank 10, displacing material radially outwardly, which appears in a matching annular bulge (not illustrated) in the outside of tank 10. On the inside, the punch edge 22 creates a shallow retention groove 24 with a depth of approximately twenty thousandths of an inch, over most of it the inner surface of tank 10. Where it approaches and opens through the side edges 12, however, the groove 24 is significantly deeper, as much as fifty thousandths of an inch, because of the D-W differential described above. Ideally, the bottom of groove 24, regardless of its depth, would reside substantially on the circular nominal perimeter shown in dotted line in FIG. 2, larger in radius only by the ideal radial brazing clearance described above. Another effect of the W-D differential prevents that ideal result, however.
Referring next to FIGS. 4 and 5, the D-W differential causes the tool working edge 22 to drag past the inner corners of the tank edges 12 with more interference than along the rest of the tank inner surface, acting to pull and draw surface metal down and toward the bottom of the tank 10. As a consequence, a localized deformation occurs both in the bottom surface of groove 24, where it opens through the side edges 12, and in the side edges 12 proximate to that opening. Specifically, the bottom of groove 24 slopes out and away from the ideal circle, potentially all the way across the width of edges 12. Also, the ideally flat surfaces of the edges 12 curve down into the groove 24.
Referring next to FIGS. 5 through 7, separator 16 is installed by dropping it into retention groove 24, and then installing a header plate 26. Header plate 26, as described above, is generally a semi-cylinder, with an accurately finished inner surface 28 having a radius substantially equal to (or only 2-4 thousandths greater than) the separator edge 18, and flat longitudinal edges 30 with a thickness of approximately seventy thousandths inch. Header plate 26 is inserted between the crimp flanges 14 until the respective pairs of edges 12 and 30 abut. The flanges 14 are then bent inwardly over the outside of header plate 26. The separator outer edge 18 is closely captured between the header plate inner surface 28 and the bottom of groove 24. However, shown by the dotted line in FIG. 7, the interface gap around the separator outer edge 18 widens from the ideal in the areas near the tank edges 12. In addition, there is a gap in the otherwise close abutment between the edge pairs 12 and 30 in the same area. When the tolerance stack ups are such as to create the worst interference condition (W at the narrowest end of the tolerance range, D at the smallest, the tool working edge 22 at its largest), these gaps can potentially allow an external leak past the seam between the edges 12 and 30, or an internal by pass leak around the separator edge 18, or both. These can be detected through proper post-braze testing, and either fixed or discarded, and routinely are. Still, it would be preferable to prevent such potential leaks, if possible.